Seems to me that biological parents ought to have pretty much absolute control over who their child is permitted to associate with, without having others impose their will on them.
Even if it is unfair.
Yours is a valid argument in favor of denying custody to the one partner. But, these lesbos who do this want it both ways. specifically:
A case in Sweden where they sued the sperm donor for child support. He lost.
Now this one. Sorry these people want to live their "lifestyle" without the responsibilities, then they hide behind or run to the courts when they change their minds. No. They made a decision, they should have to abide by the decision and not deny the woman who raised the child visitation.
Until now, you were right. Be afraid. Be very afraid.